This invention relates to a process for the preparation of alkanolamines and, more particularly, to a continuous process for preparing alkanolamines with high yields of monoalkanolamine by the reaction of alkylene oxides with a large excess of ammonia wherein the reaction mixture is maintained in a single supercritical phase.
It is known that alkanolamines, which are used in a variety of commercial applications such as emulsification agents for soaps and cosmetics and as starting materials for the production of raw materials for detergents, wetting agents, emulsifiers, textile auxiliaries and the like, can be obtained by the reaction of alkylene oxides with ammonia or amines, the yield of alkanolamines being a mixture of mono, di-, and trialkanolamines with generally equal relative proportions of the three alkanolamines being frequently obtained. The relative proportions of these three alkanolamines in the product mixture, however, are known to depend on the relative quantities of alkylene oxide and ammonia that are reacted and processes have heretofore been used or suggested for achieving higher yields of one or more of the alkanolamines in the mixture which involve varying the proportion of such reactants, such as increasing the amount of ammonia relative to the alkylene oxide to obtain increased yields of monoalkanolamine, as well as by other process changes.
There is disclosed, for example, in U.S. Pat. No. 2,196,554 to H. M. Guinot a process for preparing mono-hydroxylalkylamines with yields of 90%-95% by reacting at least 30 parts by weight of ammonia with one part of alkylene oxide in a liquid phase reaction. Relatively dilute aqueous ammonia solutions are employed and the patent discloses that steam generated during concentration of the reaction product mixture is used for heating subsequent reaction product mixtures to separate ammonia gas therefrom, thus reducing the heat energy requiremtnts for the process.
Another process for preparing alkanolamines with extremely high yields of monoalkanolamines and only small amounts of the di- and trialkanolamines by reacting alkylene oxide with large excess amounts of ammonia in a liquid phase reaction system is disclosed in U.S. Pat. No. 3,697,598 to Weibull et al. The molar ratio of ammonia relative to alkylene oxide used in the process is within the range of 10:1 to 80:1 and the reaction is carried out in the presence of a cation exchange resin catalyst. The process of the patent is described as being a continuous process which is capable of being run isothermally or, preferably, adiabatically at temperatures in the range of from 20.degree. C. to 250.degree. C. when pressures are employed that are high enough to keep the reactants and reaction products in the liquid phase throughout the reaction. There is, however, no disclosure either in the description or in the examples of the patent which suggests that high yields of alkanolamines of any type are obtained when the process is carried out either adiabatically or isothermally without the use of cation exchange resin catalysts, and patentees state that without a cation exchange catalyst it is not possible to realize an adiabatic reaction because it is too slow.
Further, in U.S. Pat. No. 3,723,530 to Goetze et al., there is also disclosed a process for preparing a mixture of alkanolamines by the liquid phase reaction of ethylene oxide and a large excess of ammonia, that is, mole ratios of ammonia to ethylene oxide of from 14 to 40 to one. The patent teaches that when the reaction is carried out in the presence of up to 1 mole of diethanolamine per mole of ethylene oxide, the product obtained will be a mixture of only monoethanolamine and triethanolamine with little or no diethanolamine being present. The process of the patent is described as being capable of being run continuously, either isothermally or adiabatically. When operated continuously, the reaction is carried out in the liquid phase at temperatures in the range of from 60.degree. C. to 150.degree. C. and pressures of from 20 to 120 atmospheres, and the monoethanolamine content of the product mixture generally does not exceed 70 percent by weight. A continuous embodiment of the process which is described as being preferred is directed to initially reacting ethylene oxide with an excess of ammonia in a liquid phase to prepare a mixture of alkanolamines from which diethanolamine is separated and recycled. The recycled diethanolamine, when added to fresh ammonia and ethylene oxide starting reactants in the proportions described, results in the net formation of reduced amounts of diethanolamine. This process, however, requires the continuous separation of diethanolamine from the product mixture and is operated in the liquid phase.
In copending application Ser. No. 195,395, filed May 12, 1988, which is a continuation of application Ser. No. 561,045, filed Dec. 13, 1983, now abandoned, and which is a continuation-in-part of application Ser. No. 247061, filed Mar. 24, 1981, now abandoned, there is disclosed a process for preparing alkanolamines with high yields of monoalkanolamines by reacting alkylene oxide with a large excess of ammonia in a single supercritical phase. The process disclosed therein is described as being capable of running batchwise or continuously under isothermal or adiabatic conditions. When the process is run continuously, a reactor which is capable of operating as efficiently as possible as a "plug-flow" reactor may be employed for carrying out the reaction. The present invention is directed to preferred embodiments of the continuously run process described in said copending application.